Microscratch test indenter

ABSTRACT

An indenter for microscratch test of durability properties of the materials, including resistance of coating films to delamination. The blade-like indenter has a prism-like body defined by a front side, a rear side, a first lateral side, and a second lateral side. The first lateral side and the second lateral side converge and form at their intersection an edge that extends from the front side to the rear side. An angle between the edge and the front side is sharp, and an angle between the first lateral side and the second lateral side is rounded with a radius. During the test, the blade-like indenter is installed at selected angles of attack to the surface of the test material; a relative movement is created between the indenter and the test material with simultaneous mechanical, electrical, and acoustical measurements. Analysis of the test results has confirmed that the micro-blade of the invention with two variable attack angles is the most effective indenter for both scratch resistance and adhesion evaluations, as compared to conventional indenters, such as sharp conical tips.

REFERENCE TO RELATED CASE

[0001] This application is divisional of the U.S. patent application Ser. No. 09/668,551 filed on Sep. 25, 2000, which is pending.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of micro-tribology, in particular to a test indenter for micro-tribological measurements of durability via microscratch and adhesion tests. The invention may find its use for studying and testing durability, wear and scratch resistance, adhesion and delamination resistance of solid surfaces, coatings and films, as well as near-surface layers of various materials, including metals, composites, polymers, ceramics, etc.

BACKGROUND OF THE INVENTION

[0003] Durability of surfaces of various materials is characterized by their wear and scratch resistance. If the surfaces are formed by coatings or films, another important characteristic of their durability is an adhesive strength with which the coating film is attached to either a substrate or an under-layer, and its delamination resistance. In combination, the aforementioned characteristics constitute a unique signature of the surface or coating.

[0004] The use of coating films, both thin and thick, in various industries is increasing constantly. Thin films are used extensively in such fields as magnetic and electronic materials. For example, a hard disk used in computer disk drives comprises either an aluminum alloy or a glass substrate coated with a multi-layered structure of various materials, including a nickel-phosphorous layer of several micron thickness, magnetic layer(s) of a fraction of micron thickness, and then a carbon overcoat less than a dozen nanometer thick. Both scratch resistance of the top carbon layer and delamination resistance, or adhesion, of each of the layers are matters of great importance for the drive durability.

[0005] Another example of thin film application is microelectronics where thin films are applied to a silicon substrate and, with photolithography and etching, are formed into well-defined fine lines used as conductive interconnections between elements of semiconductor chips. The durability of the microelectronic devices depends on the delamination resistance, or adhesion, of thin films to their substrates.

[0006] An example of a thick coating is paint, applied to various surfaces of automotive vehicles. The paint has to be scratch resistant, at the same time having good delamination resistance, or adhesion, to its metal or non-metal substrate. When paint includes two or three layers, for example an under-layer, color layer and transparent overcoat, the delamination resistance of each of the layers is an important characteristic of the durability. Another example is a coating on optical lenses, which may include anti-reflective and wear-resistant layers; durability of lenses is defined by both scratch resistance of the surface and delamination resistance, or adhesion strength, of each of the coating layers.

[0007] Therefore, there has been continued development in the art to evaluate surface durability by measuring such surface properties as resistance to scratch, or wear resistance, and resistance of coating films to delamination, or adhesive strength.

[0008] The most typical test, which finds wide application for measuring the above properties, is known as a microscratch test. It is an ideal method for characterizing the surface durability, including that of films and coatings. The microscratch test can be used for all kinds of industrial coatings from thin films in semiconductor and optical industries to decorative and protective coatings for consumer goods. The microscratch test consists in that a scratching indenter, typically either a steel or diamond conical tip or stylus, is pressed into the tested material under a known constant or progressively increasing normal load, and a relative motion is caused between the indenter and the tested surface, while evaluating the aforementioned characteristics by monitoring friction and acoustic signals.

[0009] Known in the art is a microscratch tester of CSEM type distributed by Micro Photonics, Irvine, Calif., USA. The technique involves generating a controlled scratch with a conical point indenter, either a Rockwell C diamond tip or a sharp steel tip, drawn across a coated surface under either a constant or a progressively increasing load. This is schematically shown in FIG. 1, which is a side view of the test indenter 10 on the coating 12 during the test. When the coating 12 starts to fail, the corresponding critical load is detected by means of an acoustical sensor attached to an indenter holder, evaluated in terms of a friction force between the indenter and the surface, penetration depth, and is observed with the use of optical microscopy (not shown in FIG. 1). Once known, the critical load is used to quantify the scratch resistance and adhesion properties of the film-substrate combinations.

[0010] In this test, the indenter is held perpendicular to the surface of the material being tested, and during dragging the applied load is kept normal to the test surface. A disadvantage of such point tips is that the end of the indenter is very sharp, with an extremely small radius (of about 1 to 20 μm). So, when the tip is pressed into the surface of the tested coating, it develops a very high contact pressure, and even when it does not break through the coating yet, it produces significant stresses deep in the substrate. Therefore, the test results are affected by the properties of the substrate, which makes it impossible to accurately measure the properties of thin films and coatings.

[0011] U.S. Pat. No. 5,696,327 issued in 1997 to He Huang et al. describes a microscratch test conducted with the use of a blade-type indenter, as compared with the above mentioned point microscratch test with a conical tip. In this patent, a blade-type indenter is used to facilitate calculation of the adhesion work of delamination in a two-dimensional representation, as compared to the uni-dimensional representation in the point microscratch test. The test is carried out by pressing an indenter onto a coating and moving either a blade or a test sample in relation to each other, with simultaneous application of both a normal load and a lateral force to the indenter.

[0012] The blade-type indenter used for the above test has a symmetrical wedge-shaped cross section with front and back attack angles equal to each other. The blade is held perpendicular to the tested surface and is made from a diamond or sapphire. Accuracy of determining the adhesion work is achieved by utilizing a blade of significant width, so that the data is taken from essentially macroscopic surface areas. Such blade-type indenter is not suitable for testing small local areas or thin films or multi-layered materials. Furthermore, the method of U.S. Pat. No. 5,696,327 requires preparation of special test samples with a width narrower than the width of the blade. In addition, the test data is extremely sensitive to the blade orientation, which requires ultra-precision adjustment of the cutting edge of the blade to be parallel to the tested surface.

[0013] In the known scratch test methods, only friction and acoustic measurements were combined together, whereas another known test method with measurements of electric properties (impedance, resistance, capacitance) may be carried out separately, in combination with vertical indentation test, particularly because of non-conductivity of the diamond tips used for microscratch testing. As a result, for many material combinations the exact determination of the critical load was difficult or impossible, especially in cases of thin or multi-layered coatings.

OBJECTS OF THE INVENTION

[0014] The object of the present invention is to provide a scratch test indenter, suitable for microanalysis of coatings and thin films, which allows for simultaneous precision acoustic, electrical and mechanical measurements of the indenter-coating interactions, and thus for precision determination of the critical load of, or time till, coating failure, with improved measurement data correlation, which does not produce stresses deep in the substrate under the coating, does not need preparation of special test samples, and is not very sensitive to the deviations in its position with respect to the tested surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a side sectional view of a known sharp point indenter, or tip, used for microscratch test.

[0016]FIG. 2 is a three-dimensional view of a microscratch test micro-blade indenter of the present invention.

[0017]FIG. 3 is a view of the microscratch test micro-blade indenter of the present invention in the direction of arrow A in FIG. 2.

[0018]FIG. 4 shows microscratch test data on elastomer-coated metal surfaces with a known point indenter (a) and the micro-blade indenter of present invention (b).

SUMMARY OF THE INVENTION

[0019] An indenter of the invention is intended for a microscratch test of durability of materials, including resistance of coating films to delamination. The indenter has a prism-like micro-blade shape and is defined by a front side, a rear side, a first lateral side, and a second lateral side. The first lateral side and the second lateral side converge and form at their intersection an edge that extends from the front side to the rear side. An angle between the edge and the front side is sharp, while an angle between the first lateral side and the second lateral side is rounded with a radius. For a microscratch test, the blade-like indenter is installed with a selected angle of attack between the front side and the test surface, a relative movement is created between the indenter and the test sample, with simultaneous mechanical, acoustical, and electrical measurements of the indenter-surface interactions. The results of these independent measurements are compared and analyzed for precision determination of the critical test parameters corresponding to surface (coating) failures and thus characterize the durability of the test sample by the scratch resistance of its surface and/or by delamination resistance of its layers.

DETAILED DESCRIPTION OF THE INVENTION

[0020] A three-dimensional view of the microscratch indenter of the present invention is shown in FIG. 2. The contact geometry of the blade-like indenter has been selected based on contact stress analysis and large amount of experiments. One of the chosen materials for the indenter is tungsten-carbide, since it is not only hard, but also conductive, which is critical for measuring the electric contact resistance across the interface between the indenter and the tested surface. As can be seen from FIG. 2, the microscratch indenter 30 comprises a prism-like body defined by a front side F, a rear side S, a first lateral side L and a second lateral side K. The first lateral side L and the second lateral side K converge and form at their intersection an edge RE that extends from the front side F to the rear side S. As shown in FIG. 3, which is a side view of the indenter 30 in the direction of arrow A in FIG. 2, an angle θ between the edge RE and the front side F is sharp, while an angle y between the first and second lateral sides (FIG. 2) is rounded with a radius R1.

[0021] The angle θ can be selected within the range of 10° to 160°. The angle θ can be selected within the range of 15° to 170°. Radius R1 can be within the range of 0.1 mm to 3 mm.

[0022] Although the indenter 30 is shown as a body having a uniform thickness T, this dimension is not critical, and on its side (not shown) opposite to the edge RE it may have configuration convenient for attachment to a tester.

[0023] Test geometry should be considered in combination with the angles formed between the facets or sides and the edge of the micro-blade and the surface of an object being tested. In this respect one can refer to FIG. 3, where α designates a front attack angle, which is formed between the front side F of the indenter 30, that faces the direction of movement during the test, and the surface P of the material being tested. The indenter 30 also forms a back attack angle β between its relief edge RE and the surface P. Arrow B shows a direction of movement during scratching.

[0024] The aforementioned two attack angles are crucial for successful evaluation of scratch resistance and adhesion of thin film coatings. The choice of both attack angles depends on the thickness and hardness of the coating films. The back angle β is found to be in the range of 5° to 85°, preferably from 10° to 30° for most materials. The front angle α should be in the range of 10° to 170°, preferably in the range of 60° to 120°. Both front and back angles affect the contact stress distribution and test results.

[0025] Microscratch tests were conducted for determining the durability of multi-micron-thick elastomer coatings on metal surfaces of ink-jet cartridges for printers, as well as of few-nanometer-thick carbon coatings on magnetic disks for hard disk drives. We used both the conventional sharp diamond tip of 10 micron radius and the micro-blade indenters of the present invention made of tungsten carbide and a polycrystalline diamond and having the following geometry: γ=60°, R1=0.4 mm, β=10°, α=90°.

[0026] The indenters were installed into a special holder of a micro-tribometer mod. UMT, developed and manufactured by Center for Tribology, Inc., Campbell, Calif., USA. The tester has a frame that supports a moveable carriage with guideways, having a loading unit for application of a normal load. The tester is equipped with a moving stage that supports a test material, as well as with a measuring system for simultaneous real-time monitoring and comparative analysis of a friction force F_(x), normal load F_(z), coefficient of friction COF as their ratio, electric contact resistance ECR and acoustic emission AE. In each test, the indenter was slowly moved against a coated test surface, while normal load was continuously gradually increased.

[0027]FIG. 4 presents typical results for the elastomer coatings, with the known point indenter (FIG. 4,a) and the micro-blade-like indenter per present invention (FIG. 4,b). The abscissa axes show time in seconds, the ordinate axes show a linearly increasing normal load, as well as three response signals of friction force, acoustic emission and electrical contact resistance. Under the same applied loads (FIG. 4,a—curves 1), the point indenter develops higher contact pressures and breaks the coating right away, so the loads on the point indenter had to be reduced; however, the small reduced loads (FIG. 4,a —curves 2) have led to the reduced both precision and repeatability of the measurements. Also, the absence of electrical resistance measurements did not allow for improved accuracy of determination of the critical load. Although with less accuracy, it was still possible to measure scratch resistance with the point indenter. However, it was impossible to observe coating delamination and measure coating adhesion to the substrate, as the indenter was too sharp and small to delaminate significant areas of the coating.

[0028] The microscratch test per the present invention (FIG. 4,b) allowed to clearly observe three different modes of test surface interactions with the micro-blade-like indenter. At small normal loads, the coating was deformed with no wear particles produced. Then at the critical load of scratching there was a well-defined transition from deformation to micro-scratching, after which sub-micron wear debris were produced. Then at the critical load of delamination there was another threshold, defining transition to delamination with substantial multi-micron particles being produced and chunks, or areas, of the coating coming off the substrate. All the monitored signals, namely, friction force, acoustic emission and electrical contact resistance, have fully correlated between them in both defining the moments of the thresholds and the coating modes of failure. As shown in the drawings, the interaction of the indenter with the surface of the test material was monitored as a function of time or force. Furthermore, durability and adhesive properties of the test material were evaluated by analyzing the monitored interactions.

[0029] Analysis of the test results has confirmed that the micro-blade of the invention with two variable attack angles is the most effective indenter for scratch resistance and adhesion evaluations, as compared to conventional indenters, such as sharp conical tips. The fact that the contact stress distribution produced by the micro-blade is concentrated mostly within the thin coating film, without penetrating deep into the substrate, allows to compare the scratch and delamination resistance of thin films themselves, with the minimum effect of the underlying layers or substrate. Contact stress analysis showed that with the conical point indenter, or tip, the contact stress is distributed well beyond sub-micrometer level from the surface, which is not suitable for studying processes that occur in thin films.

[0030] Thus it has been shown that the invention provides a microscratch test indenter which is suitable for microanalysis, allows for simultaneous accurate acoustic, electrical and mechanical measurements of both normal loads and friction forces with improved measurement data correlation, does not produces stresses penetrating through the coating into the substrate, does not need preparation of special test samples, and is not very sensitive to the deviations in the position of the test blade with respect to the surface of the test material.

[0031] Although the invention has been described with reference to specific embodiment, it is understood that this embodiment should not be construed as limiting the application of the invention, and various changes and modifications are possible, provided they do not depart from the scope of the patent claims. For example, the micro-blade indenter can be made of materials other than those indicated in the specification; these materials can be conductive or non-conductive, such as crystalline materials, e.g., hafnium or zirconium oxides with crystal facets sharpened at the angles corresponding to the front and back attack angles specified by the present invention. In the electrical measurements, electrical impedance or capacitance can be measured instead of resistance. 

1. A microscratch test blade-like indenter comprising a prism-like body defined by a front side, a rear side, a first lateral side and a second lateral side, said first lateral side and said second lateral side converging and forming at their intersection an edge that extends from said front side to said rear side, an angle between said edge and said front side being sharp, and an angle between said first lateral side and said second lateral side being rounded with a radius.
 2. The microscratch test blade-like indenter of claim 1, wherein said angle between said edge and said front side is within the range of 10° to 160°.
 3. The microscratch test blade-like indenter of claim 1, wherein said angle between said first lateral side and said second lateral side is within the range of 15°0 to 170°.
 4. The microscratch test blade-like indenter of claim 1, wherein said radius is within the range of 0.1 mm to 3 mm.
 5. The microscratch test blade-like indenter of claim 1, wherein said radius is within the range of 0.1 mm to 3 mm.
 6. The microscratch test blade-like indenter of claim 1, wherein said indenter is made of an electrically conductive material.
 7. The microscratch test blade-like indenter of claim 2, wherein said indenter is made of an electrically conductive material.
 8. The microscratch test blade-like indenter of claim 3, wherein said indenter is made of an electrically conductive material. 